The invention relates to digital watermarking, and more specifically relates to detecting a digital watermark in media such as images, video and audio.
Digital watermarking is a process for modifying media content to embed a machine-readable code into the data content. The data may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media such as images, audio signals, and video signals. However, it may also be applied to other types of data, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark pattern by altering data samples of the media content in the spatial or frequency domains. The reading component analyzes target content to detect whether a watermark pattern is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
One challenge to the developers of watermark embedding and reading systems is to ensure that the watermark is detectable even if the watermarked media content is corrupted in some fashion. The watermark may be corrupted intentionally, so as to bypass its copy protection or anti-counterfeiting functions, or unintentionally through various transformations that result from routine manipulation of the content. In the case of watermarked images, such manipulation of the image may distort the watermark pattern embedded in the image. In general, the distortion may result in some affine transformation of the pattern. An affine transformation encompasses various linear transformations, including scale, translation, rotation, and differential scale (e.g., shear). To accurately detect and read the watermark, it is helpful to determine the parameters of this affine transformation. The reader may then use these parameters to adjust the corrupted image to approximate its original state and then proceed to read the information content represented in the watermark.
Watermarks are often difficult to detect and read in corrupted media, particularly if the original un-marked media is not available to assist in the detection and reading process. Thus, there is a need to develop techniques for accurately detecting the presence and orientation of a watermark in corrupted media where the original media is not available.
The invention provides a method of detecting a digital watermark in media content. While the method is adapted to images, it applies to other forms of media content as well.
The watermark detection method transforms target media, suspected of having a watermark, into a selected transform domain. It then maps the transformed data into a log polar coordinate system. The method correlates the transformed data in the log polar coordinate system with a detection watermark to compute one or more orientation parameters. Having determined at least initial orientation parameters, the method may refine the correlation using these initial orientation parameters to find other orientation parameters.
Further advantages and features of the invention will become apparent with reference to the following detailed description and accompanying drawings.